Free-moving herds of migratory bison in Yellowstone National Park are stimulating the regrowth of flourishing grasslands and reshaping ecosystems, a new study has found.
Contrary to conventional grazing wisdom, the findings suggest that encouraging large-scale migrations of these massive mammals could help enhance landscapes across North America, according to the research, published Thursday in Science.
“With the current large herds of bison, Yellowstone grasslands are functioning better than in their absence,” said co-lead author Bill Hamilton, a professor in research science at Washington and Lee University, in a statement.
“This version is a glimpse of what was lost when bison were nearly wiped out across North America in the late 1800s,” Hamilton added.
Historically, the continent supported tens of millions of bison, whose seasonal migration activities transformed the region’s grasslands, the researchers noted.
But today, herds of wild, free-roaming bison are few and far between.
Driven to near extinction in the late 1800s via hunting, military operations and actions aimed at harming Indigenous communities, only about 400,000 animals remain — and most in small managed herds on private land or in parks, per the study.
While evidence has suggested bison can diversify habitats, influence plant communities and drive nutrient productivity, the broader ecological effects of their migration remain unclear due to their confined lifestyles, the authors explained.
But the restoration of bison populations in the northern portion of Yellowstone National Park provided the researchers with a rare opportunity to understand how the animals are capable of reshaping widespread landscapes.
The region has a bison population of around 5,000, having stabilized since the mid-2010s, from a low of about 23 animals in 1902, the scientists noted. The bison today tend to travel about 1,000 miles each year, in back-and-forth trajectories along a 50-mile migration route.
Along that route, the authors explained, the bison graze intensely and consume young plants that emerge following snow melts.
Between 2015 and 2022, the researchers tracked the grazing dynamics of these animals across 16 sites that were representative of their three main habitats — measuring their contributions to carbon and nitrogen dynamics, plant life and soil microbiology.
Ultimately, they discovered that as the bison grazed, they ended up accelerating the nitrogen cycle: the process by which nitrogen moves between living organisms and the environment. Grazing bison, the scientists explained, increased the presence of soil microbes, which in turn recycled nitrogen from decaying plants and animals as fertilizers.
Although the plants grew as much as they would have if they weren’t grazed, they were 150 percent more nutritious, the authors found. These more nutrient-rich plants then provided meals for other plant-eating animals.
“As bison move across the landscape, they amplify the nutritional quality and capacity of Yellowstone,” Hamilton said.
“Their grazing likely has important consequences for other herbivores and for the food web as a whole,” he added.
Hamilton compared the transformation to changes that occurred in the Serengeti after the wildebeest population recovered. In that situation, however, a fully protected migratory route now supports wildebeest at near carrying capacity, the researchers acknowledged.
“Yet bison are reshaping nutrient flow, producing effects absent or weaker in more constrained systems where herbivores are tightly regulated or fenced,” the authors concluded.
Co-author Jerod Merkle, an associate professor of migration ecology and conservation at the University of Wyoming, recognized that their results challenge prevailing notions linking overgrazing to reductions in nutrient storage.
In this case, he said in a statement, the bison provide a certain “heterogeneity” across the landscapes: they exhibit “strong variation in the amount of grazing,” with some areas cut short and others left untouched.
On a global scale, he and his colleagues expressed support for encouraging the ability of such groups of massive mammals to roam unobstructed — on landscapes “that allow for thousands of migrating large herbivores to move freely.”
“The return of a large-scale bison migration provides clear benefits to the ecosystem services that underlie Yellowstone,” Merkle added.